A boosted engine may offer greater fuel efficiency and lower emissions than a naturally aspirated engine of similar power. During transient conditions, however, the power, fuel efficiency, and emissions-control performance of a boosted engine may suffer. Such transient conditions may include rapidly increasing or decreasing engine load, engine speed, or mass air flow. For example, when the engine load increases rapidly, a turbocharger compressor may require increased torque to deliver an increased air flow. Such torque may not be available, however, if the turbine that drives the compressor is not fully spun up. As a result, an undesirable power lag may occur before the intake air flow builds to the required level.
It has been recognized previously that a turbocharged engine system may be adapted to store compressed air and to use the stored, compressed air to supplement the air charge from the turbocharger compressor. For example, Pursifull et al. describe a system in US 2011/0132335 wherein compressed air is stored in a boost reservoir and is dispensed into the intake manifold when insufficient compressed air is available from the turbocharger compressor. In particular, the boost reservoir is charged with fresh intake air and/or effluent from one or more un-fueled cylinders. By dispensing extra compressed air from the boost reservoir to the intake manifold, torque corresponding to the dispensed air can be provided to meet the torque demand while the turbine spins up.
However, the inventors herein have identified potential issues with such a system. As one example, the boost air may be consumed at a faster rate downstream of a compressor as compared to upstream of a turbine. As a result, when delivered into the intake manifold, the boost air may be initially able to supply sufficient air to provide increased desired torque, but after the supply is depleted, such as at higher engine speeds, the turbine may still not be spun up, and thus torque may drop following the initial increase. Such performance may be worse than no compensation at all. The depletion may be even faster if the boost reservoir has a small volume. Further still, if the charge in the reservoir has a large percentage of exhaust gas, delivering the boost air to the intake may not be able to compensate turbo lag due to the lack of sufficient excess oxygen in the discharged boost air.
Thus, at least some of the above issues may be addressed by a method for a turbocharged engine. In one embodiment, the method comprises, during a first tip-in, discharging pressurized charge from a boost reservoir to an intake manifold; and during a second tip-in, discharging pressurized charge from a boost reservoir to an exhaust manifold. In this way, pressurized charge is discharged into the intake or the exhaust manifold to rapidly increase exhaust temperature or pressure and expedite turbine spin-up.
For example, based on the composition of charge stored in the boost reservoir, a controller may decide whether to deliver the charge to the intake manifold or the exhaust manifold in response to a tip-in. As an example, when the boost reservoir is charged with a higher percentage of fresh air, the boost air may be applied to the intake manifold to provide increased torque to address turbo lag while the turbine spools up. In comparison, when the boost reservoir is charged with a higher percentage of combusted exhaust gas, the boost air may be applied to the exhaust manifold to enable energy from the boost charge pressure to be extracted and advantageously applied to expedite turbine spool-up. A simultaneous throttle adjustment may be performed to compensate for the increased exhaust pressure reducing the amount of air that can be inducted into the engine intake, and therefore the amount of torque delivered. For example, an opening of the throttle may be simultaneously increased to increase air inducted and torque output from the engine.
The selection of whether to discharge the boost air to the intake manifold or the exhaust manifold may also be based on a pressure of the charge stored in the reservoir. For example, if the pressure is higher than a threshold, the higher pressure charge may be discharged into the intake manifold to rapidly raise exhaust temperatures and reduce turbo lag. In comparison, if the pressure is lower than the threshold, the lower pressure charge may be discharged into the exhaust manifold to rapidly raise exhaust pressure and reduce turbo lag. Alternatively, the selection may be based on a boost level at the time of tip-in. For example, if the boost level is higher than a threshold, the boost reservoir charge may be discharged into the intake manifold, while if the boost level is lower than the threshold, the charge may be discharged into the exhaust manifold. Further still, the selection may be based on other engine operating conditions, such as engine speed, exhaust temperature, etc.
In still further embodiments, pressurized charge may be discharged to each of the intake manifold and the exhaust manifold during a single tip-in. Specifically, a portion of the charge stored in the boost reservoir may be discharged to the intake manifold and a remaining portion of the stored charge may be discharged to the exhaust manifold. In this case, the controller may decide whether to discharge to the intake manifold first or the exhaust manifold first based on the factors discussed above.
In this way, by pre-storing an amount of intake air and/or combusted exhaust gas in a reservoir and discharging into the engine intake or exhaust manifold based on operating conditions, turbo lag may be reduced even if boost is already present. Overall, engine performance may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.